Nickelocene reactions

Nickelocenes, reactions, review 27, 813 suppl. 29 Nickel phosphine complexes as catalysts 19, 778 suppl. 29 20, 531 suppl. 29 ... 

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

TC-Cyclopentadienyl Nickel Complexes. Nickel bromide dimethoxyethane [29823-39-9] forms bis(cydopentadienyl)nickel [1271 -28-9] upon reaction with sodium cyclopentadienide (63). This complex, known as nickelocene, 7T-(C3H3)2Ni, is an emerald-green crystalline sandwich compound, mp 173°C, density 1.47 g/cm. It is paramagnetic and slowly oxidi2es in air. A number of derivatives of nickelocene are known, eg, methylnickelocene [1292-95-4], which is green and has mp 37°C, and bis( 7t-indenyl)nickel [52409-46-8], which is red, mp 150°C (87,88). 

Evidence for slow inversion at nitrogen in the compound (106) and its tetrachloro analogue has been obtained 142>. The adduct of tetra-fluorobenzyne with cyclopentadiene has been obtained but of rather more interest is the isolation of the isomeric adducts (108) and (109) from the reaction of tetrafluorobenzyne with nickelocene 143>. No adducts were obtained from attempted reactions with ferrocene 144 80). 

Several preparations of silylated cyclopentadienylmetal complexes involve the formation of a triorganosilylcyclopentadienyl anion (by treatment of a silylated cyclopentadiene with an alkali metal in tetrahy-drofuran or by metalation with n-butyllithium), followed by reaction with metal chlorides. This type of reaction has been used for the synthesis of silylated ferrocenes (41, 43, 58, 83, 84, 103, 107, 116, 135, 142, 171, 172), cobaltocenes (135), nickelocene (135), titanium cyclopen-tadienyls (46, 145), and cyclopentadienylmanganese tricarbonyl (30) [Eqs. (19) and (20)]. It is remarkable that Si—C5H6 bonds are not... 

The dilithiobutane route was also successfully applied to the synthesis of the homoleptic ate complex Li2(solv)A[Ni(C4I Ix)2] (70a, solv = THF, Et20, TMEDA) (TMEDA = 7V,/V,/V, ./V,-tetramethylethylcncdiamine), which was obtained from the reaction of LiC4H8Li with either nickelocene or K[Ni(NPh2)3] [Eq. (29)].84 The permethylated derivative Li2(TMEDA)2[Ni(C4H4Me4)2] 70b was also obtained and structurally characterised (Fig. 25).85... 

Similar triple-decker sandwich ions (CsHs Mt (M = Fe or Ni) have been observed in very low abundances in the mass spectra of ferrocene and nickelocene at pressures high enough for bimolecular reactions to occur 23>. 

A reaction of the four-membered cyclic stannylene with nickelocene gave compound 102 with a Sn—Ni—Sn fragment313. 

Under certain conditions cyclopentadienyl Ugands deprotonate imida-zobum salts and can be removed from the metal center allowing the introduction of the NHC. This has been observed for some metallocene complexes such as chromocene [127] and nickelocene [128,129]. Scheme 31 shows these reactions. 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

Scheme 6.2 Proposed mechanism for the reaction of nickelocene 3 with trityl chloride...

Some of the properties of [Li(THF)2]2Ni(COD)2 (17), which can also be prepared from nickelocene, COD, and lithium, were discussed in Section III. Since (COD)2Ni(0) (2) is an intermediate in this reaction (Scheme 4), 2 can also be prepared from nickelocene by using stoichiometric amounts of lithium (42). 

The checkers found that use of a commercially available solution of nickelocene in toluene (ca. 0.27 M) is less desirable, but is admissible with certain modifications of the above procedure. If such a solution is used directly, the amount of (C6H60)3P should be increased to 225 mmoles and a reaction time of 3 days allowed, to give a yield of 53%. Alternatively, the toluene may be vacuum-stripped prior to addition of the triphenyl phosphite. Again, an excess of phosphite (325 mmoles) and a longer reaction time (24 hours) were required, giving a yield of 69%. 

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